The conventional U.S. system of oil well drilling involves the rotation of a string of drill pipe with a rotary drill bit located at the end of the drill string. Power from a motor or engine at the surface is transmitted to the bit by rotating the entire drill string. During drilling, a drilling fluid generally called drilling mud is pumped downward through the inside of the drill string and out through ports in the drill bit. The fluid then carries the material loosened by the drill bit back to the surface through the annulus between the drill pipe and the borehole. Many and varied circumstances make it desirable to drive the drill bit at speeds independent of the rotation (or stationary condition) of the drill string. A downhole motor is usually attached at or near the bottom of the drill string to accomplish this. The motor may be electric or hydraulic. If hydraulic, it may be either a turbine or a positive displacement vane or Moineau motor. All motors must have these essential elements: a power section with a stator and a rotor which produce torque and rotation between them; thrust and radial bearing supports between stationary and rotating members; a flow path for the drilling fluid from the drill string to the drill bit which path may be through the power section, and through or partially through the bearing supports. The focus of this invention is that portion of the flow path through the bearing supports adjacent to the hollow drive shaft connected to the drill bit.
Since the drilling fluid and its contaminants are hostile to the function and life of the bearings, elimination from or control of the drilling fluid through the bearings is significant to motor function, life, and overall drilling costs. Seals can eliminate the drilling fluid from the bearings and permit lubricating oil to extend bearing life. But in such application, pressure equalization means and a volume reservoir complicate design, create functional problems, which increase initial and maintenance costs. Also, effective seals are notorious for creating torque losses and expensive repairs usually result when failures occur. Control of the fluid flow through the bearings is less expensive to maintain, less subject to catastrophic failure and permits the selection of down thrust piston effect areas to more nearly balance the hydraulic down thrust with the anticipated bit loads, thereby minimizing the bearing loads which will extend bearing life. (i.e. A downhole motor is subjected to vertical loading on the thrust bearings which varies from high downward thrusts, due to the pressure of the circulating fluid on the internal working parts, to a very high upward thrust when the drill string weight on the bit exceeds the hydraulic down thrust on the shaft.)
It will be appreciated that structure to extend the operating life of the motor is extremely important in that premature failure of the motor while downhole requires expensive down time on the drill rig.